bpf: Factor callchain_store function from __bpf_get_stack

The new callchain_store function stores trace entries buffer into
user supplied buffer. It covers both just-ip and buildid data.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260803210149.296496-7-jolsa@kernel.org